1. Field of the Invention
The present invention relates to laminated balun transformers, and more particularly, to a laminated balun transformer used as a balanced-unbalanced signal converter or phase converter in a radio communication IC chip.
2. Description of the Related Art
A balun transformer converts a balanced signal in a balanced transmission line into an unbalanced signal in an unbalanced transmission line and vice versa. "Balun" is an acronym from BALanced to UNbalanced. A balanced transmission line is provided with a pair of signal paths and a balanced signal transfers, as a voltage difference, between the two signal paths. In a balanced transmission line, since external noise equally affects the two signal paths, the external noise is canceled. Therefore, a balanced transmission line is unsusceptible to external noise. Since a circuit in an analog IC chip is configured with a differential amplifier, input and output terminals for an analog IC chip signal are of a balanced type which input and output signals as voltage differences between the two terminals, in many cases. In contrast, an unbalanced transmission line transfers an unbalanced signal, as a voltage, between one transmission line and the ground (zero voltage). It includes a coaxial line and a microstripline on a substrate.
A balun transformer, in which a winding is wrapped around a magnetic core such as ferrite in a bifilar winding, is conventionally used for a balanced-unbalanced converter in a transmission line of a high-frequency circuit. Such a balun transformer, however, has a large conversion loss in a high-frequency band above the UHF band and is limited as to size reduction.
In such a frequency band, a coaxial balun transformer 51 shown in FIG. 6 is used. The balun transformer 51 has a center electrode 55, one end of the center electrode 55 being connected to an input and output terminal 52a and the other end being made open. Around the center electrode 55, two internal electrodes 56a and 56b are provided so as to electromagnetically couple with the center electrode 55. The other two input and output terminals 52b and 52c are connected to the internal ends opposing each other of the two internal electrodes 56a and 56b through leads 57a and 57b, respectively. A ground electrode 58 is provided around the two internal electrodes 56a and 56b with a dielectric member disposed therebetween. Both ends of the ground electrode 58 are connected to the external ends of the internal electrodes 56a and 56b.
Another balun transformer has also been proposed. This balun transformer is a laminated balun transformer 60 shown in FIG. 7. The balun transformer 60 includes a dielectric layer 61b on which a lead electrode 62 is provided, a dielectric layer 61c on which a .lambda./2 stripline 63 is provided, a dielectric layer 61d on which .lambda./4 striplines 64 and 65 are provided, and dielectric layers 61a and 61e on which ground electrodes 66 and 67 are provided, respectively. The .lambda./4 striplines 64 and 65 are electromagnetically coupled with the left section 63a and the right section 63b of the .lambda./2 stripline 63, respectively.
Since the balun transformer 51 of FIG. 6 has a coaxial structure, it is difficult to make it compact. Therefore, it is not suited to units such as mobile radio equipment which require a compact balun transformer.
Although the balun transformer 60 of FIG. 7 is definitely more compact than the balun transformer 51 having the coaxial structure, since the .lambda./2 stripline 63 is routed on the dielectric layer 61c, the balun transformer 60 occupies a large area on a printed circuit board when it is mounted on the printed circuit board.
To adjust the electric characteristics of the balun transformer 60, electromagnetic coupling between striplines is adjusted by changing the thickness of a dielectric layer and the width of a stripline. However, there is no other way but to change the width of the .lambda./4 stripline 64 or the line width of the left-hand section 63a of the .lambda./2 stripline 63 to, for example, independently adjust electromagnetic coupling between the .lambda./4 stripline 64 and the left-hand section 63a of the .lambda./2 stripline 63 and electromagnetic coupling between the .lambda./4 stripline 65 and the right-hand section 63b of the .lambda./2 stripline 63. This is because, when the thickness of the dielectric layer 61c disposed between the .lambda./4 striplines 64 and 65 and the .lambda./2 stripline 63 is changed, electromagnetic coupling between the .lambda./4 stripline 65 and the right-hand section 63b of the .lambda./2 stripline 63 is affected. Adjustment by stripline width causes a slight change and it is not easy to adjust electromagnetic coupling between striplines.